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While searching around, I kept hearing that fusion was a good choice, but how best to covert the energy to electricity was an issue.

However, during my research, I came across this:

Photon-intermediate direct energy conversion (PIDEC) is somewhat similar to a concept of fluorescent light - as in the CFL, in the nuclear reactor the original type of energy generated is not useful to humans. CFL uses a fluorescent coating on the inside of the light bulb to convert that energy into visible spectrum of the light. PIDEC uses fluorescer (in the form of gas) surrounding nuclear fuel acting as photon producer - fluorescer gets excited by neutron emissions and in turn emits narrow band ultraviolet light. That light is then relatively easily converted into electricity by special photo-voltaic converter.

Because the photons emitted by fluorescer are narrow band, the conversion efficiency is much higher than efficiency of common solar cells. The overall efficiency of PIDEC is expected to be around 40%. The remaining residual heat is still high enough to use it in traditional thermalized way via Carnot Cycle e.g. steam turbine. A combined efficiency of such conversion system (PIDEC + traditional) could reach as much as 70%. In comparison, due to limitations of using solid nuclear fuel and water as coolant, current generation of nuclear plants average only about 35% conversion efficiency.

Now as explained here, PIDAC was conceived for use in high temperature fusion reactors.

In addition, it is shown as capable of also generating power from fission and radioisotopes

During my research into this concept I came across this study: https://www.sciencedirect.com/science/article/pii/014919709090003N

A two-step photon-intermediate technique for the production of electricity, chemicals or lasers in nuclear energy conversion

In addition to electric power, photolysis makes other product forms possible. These products include useful feedstock, or combustion chemicals, such as hydrogen and carbon monoxide, and excited molecular and atomic states, used for laser amplifiers or oscillators.

Interesting...I mention this as it reminds me of how Mark Simmons describes how Gundams and Zakus generate both power, propulsion, and even their beam weaponry: http://www.ultimatemark.com/gundam/power.html

Since electrical generation, propulsion, and cooling all involve extracting thermal energy from the reactor and transferring it to other parts of the mobile suit's body, why not use the same mechanism for all three? The author imagines a network of thermal energy conduits running throughout the mobile suit's body, transferring reactor heat via high-pressure helium gas (8). This provides a handy explanation for the cables and tubes that decorate the exteriors of our favorite mobile suits (9).

The classic MS-06 Zaku II. In the author's opinion, its trademark power cables are most likely used to transfer thermal energy for propulsion, electrical generation, and cooling purposes. This theory is, however, at odds with the official explanation (10).

Here are his footnotes fyi:

(8) Entertainment Bible 1: One Year War Picture Encyclopedia identifies helium as the coolant used in the MS-06 Zaku II. Since this is also a suitable medium for transferring reactor heat to generator turbines, and its relatively low molecular weight makes it an efficient propellant for thermonuclear rocket engines, it seems like a good candidate for all three applications. Plus, this would explain the significance of the mysterious "helium control cores" attached to the Gundam's skirt armor!

(9) For example, the Master Grade GM Custom kit manual explains that the cables that run down the back of the mobile suit's legs supply energy to its leg thrusters.

(10) The official explanation for the Zaku II's cables is that they transmit hydraulic power to actuators in the mobile suit's joints. This always struck me as absurd, for why would the Zaku need to transmit hydraulic power from its belly to its backback, or from its muzzle to the back of its head? Likewise, early mobile suits like the Zaku II and the Gundam are usually said to use traditional chemical rocket engines, but this seems to defeat the purpose of having a thermonuclear reactor in the first place.

Now while Mark's explanation makes sense, it seems to me that PIDAC fulfils a very similar design.

For example a similar form of propulsion to Mobile Suits using PIDEC would be based on the concept: "Nuclear Lightbulb" concept

As someone else put it: https://space.stackexchange.com/a/27372

The idea is that you operate a fission reactor in a gas (really plasma) phase inside a transparent pressure vessel. The fissionables might be mixed with a fluorescing compound. If you run the reactor hot enough, radiation (which scales with the fourth power of temperature) becomes the dominant mode of energy transfer, primarily in the form of UV light. You pass your reaction mass - likely hydrogen doped with something to improve its UV absorption - over the outside of the reactor vessel. It's heated by the UV, conceptually to much higher temperatures than possible with solid core NTRs.

The concept hinges on the reactor vessel being so perfectly transparent to UV radiation that you can pass gigawatts of UV light through it without it absorbing them and therefore heating and melting. Additionally you need to run a plasma-phase fission reaction inside it (good luck with your neutron economy) and somehow protect it from that ferociously hot and corrosive material.

I am pretty sure he isn't right about the reaction mass needing to be passed over the outside of the vessel. It's all internal. At least it hasn't come up in the official scientific documentation. He also mentions heat would be a problem, but again thermal conversion is comparable with PIDEC. Not to mention there seems to be a way around the radiation damage issue

Back to Mark's article

However, this doesn't account for all of the cables. Mobile suits also need to transmit beam energy, in the form of Minovsky particles, to their weapons. The Gundam's beam rifle and beam saber both contain energy capacitors, which store the high-energy Minovsky particles used to form their devastating beams. In the case of the beam rifle, electrical power from the mobile suit's generators is used to convert the stored Minovsky particles into massive, fast-moving mega particles prior to firing.

Interesting isn't it. Especially since PIDEC also seems to be very useful for the making of Nuclear-Pumped lasers under a similar principle. (See p.107 in the google book link for reference)

I do think that if we ever make fusion powered mecha, this would be the best option, especially with recent breakthroughs in photovoltaics: https://www.nature.com/articles/ncomms14962

Is this energy conversion scheme plausible?

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    $\begingroup$ That's a huge wall of text... First of all, define best: a Lamborghini Camargo can be the best on German highways, but drop it in a Dutch canal and see how it fares. Then, give us a problem to solve. We give solutions, not opinions/thoughts. $\endgroup$ – L.Dutch - Reinstate Monica Jan 23 at 9:42
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    $\begingroup$ I'd love to see how hydrogen could be "doped". Perhaps your question could ask about the limitations of a proposed PIDEC system re size, mass, longevity, fragility, suitability, maintenance etc.. $\endgroup$ – BLT-Bub Jan 23 at 9:50
  • $\begingroup$ That came from another guy who I don't think is as smart as he thinks he is. $\endgroup$ – Jacob Blaustein Jan 23 at 10:21
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    $\begingroup$ I also note that you have 5 other questions over the last six months, some of which have highly upvoted answers, none of which you've accepted. It would be nice if you accepted some or more of those answers, or updated your questions to state why the existing answers are inadequate. $\endgroup$ – Starfish Prime Jan 23 at 13:00
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    $\begingroup$ @StarfishPrime It's making a lot more sense now, that paper's a good find. Thanks. $\endgroup$ – BLT-Bub Jan 23 at 16:02
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So, crawling through that wall-o-text, you seem to be proposing a closed-cycle gas core nuclear reactor with PIDEC to provide power for a mecha, right?

Lets ignore the weirdness about "transferring thermal energy", it isn't a totally daft idea on the face of things, but:

  1. Nuclear fission reactors have a minimal size. The "nuclear lightbulb" rocket designs weigh about 30 tonnes, and were about six metres long.
  2. Thats excluding heatsink mass, and those things will generate a lot of waste heat. Heatsinking is the bane of realistic spaceship designs, and nuclear giant mecha are no different. Nuclear powered (war)ships can use the sea. The atmosphere isn't nearly as good.
  3. It also excludes radiation shielding, and those things are going to be producing quite a lot of that.
  4. You can't just turn a nuclear lightbulb off on a whim. And when it is off, you can't just flip a switch and have it come back to life again. This might not bother you.
  5. You're building something intended to go into combat, and inside it you have a giant glass bottle full of super high pressure uranium plasma. When one of these things goes boom, and eventually one of them will, it'll be Really, Really Bad News for the local environment, and anyone downwind.

I am pretty sure he isn't right about the reaction mass needing to be passed over the outside of the vessel. It's all internal.

Don't confuse reaction mass (the working fluid of your rocket or reactor, that you heat up and expel out of a nozzle or run through a turbine) with fuel (the fissile gas on the inside of the lightbulb). In a closed-cycle gas core rocket (or reactor) the working fluid is kept entirely separate.

If you want to use PIDEC, then there is no working fluid to heat (because the UV emitted by the nuclear plasma is directly used by your PIDEC system) but you'll still need to cool the damn thing, and for that you'll want to be running large quantities of high heat-capacity and UV-transparent coolant over the outside of the lightbulb.

Is this energy conversion scheme plausible?

It is plausible, but not practical. Too much heat, too much radiation.

Honestly, your best bet will be some kind of net-energy-gain hydrogen-boron-11-fuelled athermal fusor. Such a thing may as well be handwavium at this point, but doesn't appear to be impossible... an example of such a device would be a dense plasma focus. Certainly, dense plasma focus devices exist now, though they're no good as power sources, but to my knowledge no-one has ever made something that involves controlled fission in a uranium plasma! The minimum size is much lower than a fission lightbulb, the amount of neutral particle radiation is vastly less. Depending on how efficiently you can convert x-rays to electricity, the heat output for a given power output might be less, too. That is something you might be able to do with PIDEC if you felt the need, though I suspect it'll run hot. PIDEC is less handwavium than x-ray-voltaics, at least.

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  • $\begingroup$ Thank you for suggesting the dense plasma focus. I did kinda imagine how the reactor would have to compress the fuel source and released energy. $\endgroup$ – Jacob Blaustein Jan 26 at 10:24
  • $\begingroup$ I am confused by your concerns though. Radiation wouldn't be an issue: sciencedirect.com/science/article/abs/pii/S0969804317308564 & as for the heat I would combine the PIDEC w/ radioisotope thermoelectric generators. As for the heat sinks, I would take a page from old school Super Robot Anime and use them to channel the heat towards my foes... are you sure the radiation would still be a problem? I could add betavoltic packs if necessary $\endgroup$ – Jacob Blaustein Jan 26 at 10:33
  • $\begingroup$ Sorry, scratch the betavoltic packs. As I pointed out in the op, PIDEC is also capable of generating power from fission and radioisotopes: researchgate.net/publication/… $\endgroup$ – Jacob Blaustein Jan 26 at 11:00
  • $\begingroup$ @JacobBlaustein of course radiation is an issue: you're building a nuclear reactor. You have to shield all your electronics, and the pilot, and any nearby friendlies. You're going to get all sorts of unpleasant radiation shooting out. Of course your power extraction hardware needs to be radiation resistant, but how are you going to do the same with the meatbag pilot? Or a complex computer-based AI pilot for that matter? Unless your power extraction mechanism can stop all radiation coming out (and spoiler alert: if it does, it'll be way too big and heavy) it will remain a problem. $\endgroup$ – Starfish Prime Jan 26 at 12:36
  • $\begingroup$ I think we do have a mechanism for just this sort of thing: ncbi.nlm.nih.gov/pubmed/29729484 several in fact: mospace.umsystem.edu/xmlui/handle/10355/769 The radiation is converted by the fluorescer medium into photons. It's how it's supposed to work. It's well explained in the links $\endgroup$ – Jacob Blaustein Jan 26 at 22:56
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Have you considered a decentralized array of Radioisotope thermoelectric generators?

However, to move giant mechas you would also need really big and efficient batteries to bypass the usage spikes. Fusion or fission IS possible, but cooling the thing down is an engineering nightmare. If done by gas, it would not be for impulse, since less gas = less cooling. And thermal energy can't be used for impulse. To gain momentum you need to launch something in the opposite direction, unless you are resorting to technobabble or magic, which is not bad per se, but lacks the reality barnish.

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  • $\begingroup$ RTGs are best suited for providing small amounts of power over long periods of time (i.e. decades). Mechas require the exact opposite: large amounts of power for very short periods. I'd remove that line and just concentrate on the second paragraph. $\endgroup$ – F1Krazy Jan 23 at 15:01
  • $\begingroup$ If you're going with RTGs, you'll need batteries, at which point those batteries could have been charged by any source of electricity, not just an RTG. $\endgroup$ – Mark Storer Jan 23 at 15:51

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